Solid state ultracapacitor

ABSTRACT

An ink of the formula: 60-80% by weight BaTiO 3  particles coated with SiO 2 ; 5-50% by weight high dielectric constant glass; 0.1-5% by weight surfactant; 5-25% by weight solvent; and 5-25% weight organic vehicle. Also a method of manufacturing a capacitor comprising the steps of: heating particles of BaTiO 3  for a special heating cycle, under a mixture of 70-96% by volume N 2  and 4-30% by volume H 2  gas; depositing a film of SiO 2  over the particles; mechanically separating the particles; incorporating them into the above described ink formulation; depositing the ink on a substrate; and heating at 850-900° C. for less than 5 minutes and allowing the ink and substrate to cool to ambient in N 2  atmosphere. Also a dielectric made by: heating particles of BaTiO 3  for a special heating cycle, under a mixture of 70-96% by volume N 2  and 4-30% by volume H 2  gas; depositing a film of SiO 2  over the particles; mechanically separating the particles; forming them into a layer; and heating at 850-900° C. for less than 5 minutes and allowing the layer to cool to ambient in N 2  atmosphere.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made in the performance of work undera NASA contract and by an employee of the United States Government andis subject to the provisions of Public Law 96-517 (35 U.S.C. §202) andmay be manufactured and used by or for the Government for governmentalpurposes without the payment of any royalties thereon or therefore. Inaccordance with 35 U.S.C. §202, the contractor elected not to retaintitle.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to the field of capacitors and moreparticularly to the field of solid state ultracapacitors.

(2) Description of the Related Art

Electrical, electronic, and electromechanical (EEE) parts are used manyproducts. Better energy storage and delivery devices are currentlyneeded. For example, space vehicles use rechargeable batteries thatutilize silver zinc or lithium-ion electrochemical processes. Thesecurrent state-of-the-art rechargeable batteries cannot be rapidlycharged, contain harmful chemicals, and wear out early. A solid-stateultracapacitor is an EEE part that offers significant advantages overcurrent electrochemical and electrolytic devices.

Ultracapacitor behavior has been reported in a number of oxides,including reduced barium titanate (BaTiO₃ 40) and ferroelectricceramics. BaTiO₃ 40 is a ceramic material in the perovskite family thatpossesses a high dielectric constant. Individual coating offerroelectric BaTiO₃ 40 grains with a silica (SiO₂ 48) shell, followedby spark plasma sintering (SPS) in reducing conditions, has been shownto lead to stable ultracapacitor behavior. The permittivity values havebeen reported to be ≈10⁵ in electroceramics. It has also been shown thattreating oxidized BaTiO₃ 40 at high temperatures in reducing forming gasatmosphere (75-96% nitrogen, N₂, and 4-25% hydrogen, H₂) produces anN-type semiconducting material. The outer coating, which remains aninsulating shell, combines with this semiconducting internal layer,resulting in millions of nanocapacitors in parallel. The combination ofa semiconducting grain with an insulating boundary leads to the IBLCeffect.

These so-called giant ultracapacitor properties are not easilycontrolled. American Piezo Ceramics International reports a relativedielectric constant of 1,550 and a dielectric dissipation factor (DF) of0.5 for single-crystal BaTiO₃. High permittivity values such as 10,000are reported in polycrystalline ferroelectric BaTiO₃. Reduced BaTiO₃ 40of grain sizes between 70 nm and 300 nm have yielded colossalpermittivity values on the order of ≈10⁵ The instant invention wasdeveloped by evaluating shell-coated BaTiO₃ 40 processed under reducingconditions to produce the IBLC effect.

BACKGROUND List of Acronyms and Symbols

-   -   ALD atomic layer deposition    -   Al₂O₃ alumina    -   BaTiO₃ barium titanate    -   DF dissipation factor    -   EDLC electrochemical double-layer capacitor    -   EEE electrical, electronic, and electromechanical    -   ESR equivalent series resistance    -   H₂ hydrogen    -   HESSCap high-energy, solid-state capacitor (module)    -   IBLC internal barrier layer capacitor    -   LCR inductance, capacitance, and resistance    -   N₂ nitrogen    -   SCFH standard cubic feet per hour    -   SEM scanning electron microscope    -   Si silicon    -   SiO₂ silica    -   SPS spark plasma sintering    -   STEM-EDS scanning transmission electron microscopy-energy        dispersive spectroscopy    -   Ti titanium    -   A electrode surface area    -   C capacitance    -   C_(total) total capacitance    -   D distance between plates    -   E energy    -   e electron    -   f frequency    -   I current    -   i imaginary unit    -   J/cc Joules/cubic centimeter    -   P power    -   P_(max) maximum power    -   Q stored charge    -   t resistance    -   t time    -   tan δ loss tangent delta    -   V voltage    -   V₀ vacancy site    -   V_(f) final voltage    -   V₀ initial voltage    -   X reactance    -   Z impedance    -   ∈₀ vacuum permittivity    -   ∈_(eff) effective permittivity    -   ∈_(r) dielectric permittivity

Conventional Capacitors

A capacitor 6 is an electrical component consisting of two conductingelectrodes 4, 20 separated by an insulating dielectric material,typically air 24. When voltage is applied across the capacitor 6,opposite charges accumulate on the surface of each electrode, developinga static electric field. This field causes atoms in the insulator 24 topolarize, producing an internal electric field. Capacitors 6 are able tostore energy in this overall electric field. This is illustrated in FIG.1.

Capacitance (C) is a measure of the ability to store charge, and it isthe ratio of the stored charge (Q) to the applied voltage (V):

$\begin{matrix}{C = \frac{Q}{V}} & (1)\end{matrix}$

A specific material polarizes in response to an electric field. Thevacuum permittivity (∈₀) is a constant due to free space vacuum and is8.854187 . . . ×10⁻¹² F/m. The relative permittivity multiplied by thevacuum permittivity is usually called the effective permittivity(∈_(eff))

$\begin{matrix}{C = {ɛ_{0}ɛ_{r}\frac{A}{D}}} & (2)\end{matrix}$

Capacitive loads oppose the change of voltage. Impedance (Z) is ameasure of the effect of capacitive loads. When reactance (X) is zero,the load is purely resistive; when resistance (R) is zero, the load ispurely reactive. Ideal capacitors 6 consist entirely of reactance,having infinite resistance:

$\begin{matrix}{{Z = {R + {j\; X}}}{and}} & (3) \\{X_{C} = \frac{1}{2\;\pi\;{fC}}} & (4)\end{matrix}$

Loads are modeled as either a series or parallel combination of aresistive and a reactive load. The parallel resistance is typicallylarger than the series resistance. To measure small reactive values,such as high-valued capacitors 6, it is preferable to use the seriesmodel because the series resistance is more significant than theparallel resistance. When measuring large reactive values such ashigh-valued inductors or low-valued capacitors 6, it is preferable touse the parallel model. Table 1 shows the capacitance ranges and whichmodel should be used.

TABLE 1 Model for corresponding capacitance ranges. Type of MeasurementRange Impedance Model Capacitance >100 μF <10 Ω Series Capacitance 10 nFto 100 μF 10 Ω to 10 k0 Series or parallel Capacitance  <10 nF  10 k0Parallel

At low frequencies, a capacitor 6 is an open circuit, as no currentflows in the dielectric. A DC voltage applied across a capacitor 6causes positive charge to accumulate on one side and negative charge toaccumulate on the other side; the electric field due to the accumulatedcharge is the source of the opposition to the current. When thepotential associated with the charge exactly balances the appliedvoltage, the current goes to zero. Driven by an AC supply, a capacitor 6will only accumulate a limited amount of charge before the potentialdifference changes polarity and the charge dissipates. The higher thefrequency, the less charge will accumulate, and the smaller theopposition to the current.

The two primary attributes of a capacitor 6 for this invention are itsenergy density and power density. The energy (E) stored in a capacitor 6is directly proportional to its capacitance:E=½CV ²  (5)

To determine power, capacitors 6 are represented in series with anexternal load resistance (R), as shown in FIG. 1. The internalcomponents of the capacitor 6 itself contribute to the resistance as theequivalent series resistance (ESR). Maximum power (P_(max)) for acapacitor 6 occurs at matched impedance (R=ESR):

$\begin{matrix}{P_{\max} = \frac{V^{2}}{4 \times {ESR}}} & (6)\end{matrix}$

ESR is an AC resistance dependent on frequency. In nonelectrolyticcapacitors 6 such as electroceramics, the resistance of the leads andelectrodes and losses in the dielectric cause the ESR. For a capacitor6, the ESR typically falls between 0.001 and 0.1 Q and is desired to below. A high ESR causes increased heat dissipation and results inaccelerated aging under high temperature and large ripple currentconditions. Additionally, capacitors 6 exhibiting high ESR have a highcurrent leakage, consuming and wasting power in the idle state, makingthem bad energy storage devices:P=I ² ×ESR.  (7)

Electrical potential energy is dissipated in dielectric materials in theform of heat. The DF is a measure of loss rate of energy and isproportional to the ESR. Dissipation factor is also known as losstangent delta (tan S), and it is represented as a percentage. Thisparameter depends on the dielectric material and the frequency of theelectrical signals. In high dielectric constant ceramics, DF can be1%-2%:

$\begin{matrix}{{\tan\mspace{11mu}\delta} = {\frac{ESR}{X_{C}} = {DF}}} & (8)\end{matrix}$

Electrical characteristics of ultracapacitors today lie between those ofaluminum-electrolytic capacitors and fuel cells. The electrochemicaldouble-layer capacitor 10 (EDLC) (FIGS. 2 A and B) uses high surfacearea electrodes 4, 20, resulting in ultracapacitor behavior. EDLCs 10are constructed from two carbon-based electrodes 4, 20, an electrolyte22, and a separator 26. Ions within the electrolyte solution 22accumulate at the surface of the electrodes 4, 20 and the separator 26creates a double-layer of charge. EDLCs 10 generally operate with stableperformance characteristics for many charge-discharge cycles, sometimesas many as 10⁶ cycles. On the other hand, electrochemical batteries aregenerally limited to only about 10³ cycles. Because of their cyclingstability, EDLCs 10 are well suited for applications that involvenonuser-serviceable locations. Examples include deep sea and mountainenvironments. However, EDLCs 10 cannot be used in aerospace environmentswithout hermetically sealed containers, which increase mass and volume.Currently, electrolytic ultracapacitors are used primarily inconjunction with batteries in terrestrial environments to capture suddenbursts of energy (e.g., regenerative braking systems). However,electrolytic ultracapacitors do not possess the energy density necessaryto replace batteries.

Dielectric tan δ of ceramic capacitors is dependent upon specificcharacteristics of the dielectric formulation, level of impurities, aswell as microstructural factors such as grain size, morphology, porosityand density.

Electrochemical Double-Layer Capacitor

Conventional capacitors have relatively high power densities but lowenergy densities when compared to electrochemical batteries. Statedanother way, a battery may store more energy but cannot deliver it asquickly as a capacitor can. Current ultracapacitors exploit high surfacearea electrodes and thin dielectrics to increase both capacitance andenergy. Additionally, ultracapacitors have advantages overelectrochemical batteries and fuel cells, including higher powerdensity, shorter charging times, longer cycle life and longer shelflife. The Ragone chart in FIG. 3 compares the power and energy densitiesof different types of current energy storage devices.

Internal Barrier Layer Capacitor

A solid-state ultracapacitor would overcome the limits of both theelectrochemical batteries presently being used and of currentlyavailable electrochemical ultracapacitors.

Solid-state ultracapacitors provide a robust energy storage device withhigher reliability, less weight and less volume than electrochemicalbatteries and electrolytic ultracapacitors. They are recyclable energystorage devices that offer higher power and a greater number ofcharge/discharge cycles than current rechargeable batteries. They alsooffer greater breakdown voltage than current electrolyticultracapacitors. The instant invention is a high-energy, solid-statecapacitor (HESSCap) module to replace batteries and currentstate-of-the-art ultracapacitors. Table 2 presents the primaryparameters for aerospace batteries, terrestrial electrolyticultracapacitors, and the target values for the HESSCap.

TABLE 2 Ultracapacitor/battery comparison. Energy DensityCharge/Discharge Voltage Device (J/cc) Cycles (V) Aerospace battery(Li-ion) 172  500-2,000 28 Aerospace range safety 57    <12 28 battery(Ag Zn) Commercial electrolytic 15 >500 with 50% V 59 ultracapacitor and25% C decrease ES43 solid-state module (28 V) 80-200 >500,000 28

The HESSCap module achieves high permittivity via the IBLC effect, shownin FIGS. 4A, B and C, individual ferroelectric grains are coated by adielectric shell, followed by sintering at high temperatures underreducing forming gas atmosphere (96% N₂ and 4% H₂). The forming gaspenetrates the shell and reacts with the inner grain, making each grainsemiconductive. The coating serves as an insulator, resulting inmillions of nanocapacitors in parallel:C _(total) =C ₁ +C ₂ + . . . +C _(n)  (9)

The two main parameters for the internal barrier layer to increase theoverall dielectric permittivity of oxides are (1) the inner grainconductivity and (2) the insulating grain boundary. The former isrelated to the amount of charged defects intentionally formed during thesintering step under reducing conditions. The IBLC model can be appliedto any material where extended dielectric interfaces of very smallthickness separate (semi)conducting parts: in ceramics, insulating grainboundaries surround conducting grains; in thin films and multilayers,surfaces and intergrowth planes can induce dielectric barriers betweenconducting layers. However, the exact nature of the conduction mechanismwithin the grains and of the charge accumulation at the grain boundariesis not well understood.

In Appl. Phys. Lett., Vol. 94, No. 7, 3 pp., doi: 10.1063/1.3076125,February 2009, Chung, U.-C.; Elissalde, C.; Mornet, S.; et al.(hereafter Chung) disclose controlling the internal barrier in low lossBaTiO₃ supercapacitors.

Chung discloses “standard BaTiO₃ particles of 500 nm diameter” that“have been individually coated with a homogeneous amorphous silica shellof 5 nm thickness using a method derived from the Stöber process.”However, the instant invention utilizes a proprietary gas-phase chemicalprocess rather than the Stöber process. Chung also discloses sinteringin a “reducing atmosphere at a final temperature of 1100° C. undervacuum”.

Chung further states that the density of the pressed pellets was 97%.Chung states that “the room temperature dielectric permittivity is inthe range of the so called giant dielectric materials (∈˜2.105 at f=10⁴Hz) meaning that our core shell particles are indeed leading to IBLCceramics (FIG. 2a ).”

Chung reports that the dielectric losses of the material are on theorder 5% at 10⁴ Hz “instead of 100% in the existing literature”. TheBaTiO₃ particle size and thickness of the silica shell of Chung areidentical to the particle size and coating thicknesses for SiO₂ for theinstant invention. However, Chung does not disclose a capacitorfabricated utilizing the disclosed IBLC ceramic material. Also, Chungdoes not disclose the proprietary coating process utilized in thisinvention. Further, the (Chung sintering temperature 1100° C.) issomewhat higher than the 850-900° C. temperature utilized to fabricatethe capacitors in accordance with the instant invention.

Reynolds et al. U.S. Patent Publication No. 2014/0022694 (hereafterReynolds), discloses a method for manufacturing multi-layer ceramiccapacitors. At paragraph [0058], Reynolds discloses a method includingforming a bottom electrode on a substrate utilizing “thick film methodssuch as screen printing or tape casting” or “thin-film techniquesincluding but not limited to sputtering, evaporation, ion plating, postlaser deposition, atomic layer deposition, chemical vapor deposition,plasma-enhanced chemical vapor deposition, electroplating andelectroless plating.”

At paragraph [0060], Reynolds states that the ceramic dielectric isdeposited following deposition of the bottom electrode which “can be bythick film techniques such as screen printing or tape casting.” Reynoldsstates that thin-film techniques can also be utilized to deposit theceramic dielectric. At paragraph [0060], Reynolds discloses apost-deposition heat treatment of the ceramic dielectrics such as “ahigh temperature firing in vacuum or in a reducing environment to removethe organic and volatile compounds of the inks and binders used, forexample, in the screen printing process and also to form the desiredcrystal and grain structures for high-k materials such as doped bariumtitanates that must be converted to their perovskite phase.”

At paragraph [0061], Reynolds states that the ceramic is then coatedwith a thin film “such as silicon nitride (SiN_(x)), silicone dioxide(SiO₂), aluminum oxide (Al₂O₃), etc.” Reynolds states that the films“will probably have thicknesses > to 5 nm” but “they can be eventhinner.” Suitable deposition techniques “include sol-gel deposition,sputtering, evaporation, ion plating, pulse laser deposition, atomiclayer deposition, plasma-enhanced chemical vapor deposition,‘electrografting’ and especially chemical vapor deposition.”

At paragraph [0061], Reynolds also states that “atmospheric CVD is againpreferred because thermal CVD is able to penetrate into very smallspaces, even between the gaps of the individual high-k grains. In thisway, an internal barrier-layer type capacitor dielectric is formed witha large capacitance but with reduced leakage and increased dielectricbreakdown strength.”

At paragraph [0062], Reynolds states that the substrates can beintroduced into a multi-zone furnace having a first high temperaturezone incorporating a reducing ambient. The stack is then allowed tocool, and a second layer of metal electrode material is deposited, and asecond layer of high-k ceramic is then deposited onto the second metalelectrode (paragraphs [0063]-[0064]).

However, in the instant invention the particles used to formulate thedielectric ink are coated with a proprietary gas-phase chemical process.In contrast, Reynolds states that an internal barrier-type capacitordielectric can be formed by utilizing a CVD process to coat particles(e.g. barium titanate 40) after the particles are deposited using an inkand screen printing technique. This results in only an upper layer ofparticle coating whereas inner particles are not coated.

Development of a capacitor for replacing batteries which can providelonger life, lower mass-to-weight ratio, rapid charging, on-demand pulsepower, improved standby time without maintenance, and environmentalfriendliness represents a great improvement in the field of electronicsand satisfies a long felt need of engineers and manufacturers.

SUMMARY OF THE INVENTION

The present invention is a novel method for forming solid stateultracapacitors, utilizing internal barrier layer capacitor (IBLC)material. IBLC materials generally include electrically conductivegrains that are coated by an insulating material. Ferroelectric grainsmay be coated by a dielectric shell, followed by sintering at hightemperatures in a reducing forming gas atmosphere (96% N₂ and 4% H₂).The forming gas penetrates the shell and reacts with the inner grain,making each grain semiconductive. The coating serves as an insulator,resulting in millions of nanocapacitors in parallel.

The instant invention is a method of manufacturing a capacitor.Particles of BaTiO₃ having an average grain diameter of 100-700 nm arefirst heated in a furnace under a mixture of 70-96% by volume N₂ 4-30%by volume H₂ gas for 60-90 minutes at 900° C.

The first furnace may be a multizone belt furnace or a fluidized bedvertical tube furnace. The second furnace may be a multizone beltfurnace.

Next a 3-20 nm film of SiO₂ or Al₂O₃ is deposited over the particles.The resulting material is agglomerated so the coated particles must bemechanically separated. The particles are then incorporated into an inkof the following formulation:

-   -   i. 60-80% by weight separated, coated, treated ceramic        particles;    -   ii. 5-50% by weight high dielectric constant glass; the high        dielectric constant glass being 0.5-10 μm in size    -   iii. 0.1-5% by weight surfactant;    -   iv. 5-25% by weight solvent; and    -   v. 5-25% weight organic vehicle.

Next a layer of the dielectric ink is deposited on a substrate with apre-sintered and deposited electrode in place. Methods of depositingelectrodes on a substrate are well known in the field of capacitormanufacture. The electrode can be silver, silver palladium, or anymaterial with a resistance between 1 milliohm and 10 ohms. Finally, thedielectric ink is sintered onto the substrate by heating in a secondfurnace at 850-900° C. for 60-90 minutes and allowing the ink andsubstrate to cool to ambient. This heating and cooling cycle is carriedout under N₂ atmosphere, which contains less than 25 ppm 02. Preferably,during sintering, the time under 600° C. is kept to 30 minutes maximum;time under 800° C. is 20 minutes maximum; and total time is 60-90minutes. Also, preferably, the heating rate is 45-55° C./minute from300-500° C.; and the cooling rate is 45-55° C./minute from 700-300° C.Once the dielectric is sintered, a top electrode is added that can besilver, silver palladium or any material with a resistance between 1milliohm and 10 ohms.

The substrate is preferably 0.025-0.040 inch thick Al₂O₃ in which theAl₂O₃ is at least 96% pure. Alternatively, the substrate could bealuminum nitride (AlN) zirconia, beryllium oxide (BeO) or uncuredceramic. Uncured ceramic is a mixture of ceramic particles, a binder, asurfactant and a solvent. Formulae for uncured ceramic are well known.

Preferably the thickness of the deposited dielectric ink is sufficientto produce a sintered layer 10-35 μm thick.

This invention is also an ink of the following formulation:

-   -   a. 60-80% by weight BaTiO₃ particles coated with a 3-20 nm film        of SiO₂ or Al₂O₃; the BaTiO₃ particles having an average grain        diameter of 100-700 nm; the BaTiO₃ having doubly ionized oxygen        anion vacancies;    -   b. 5-50% by weight high dielectric constant glass; the high        dielectric constant glass being 1-10 μm in size;    -   c. 0.1-5% by weight surfactant;    -   d. 5-25% by weight solvent; and    -   e. 5-25% weight organic vehicle;

The solvent may be ester alcohol, terpineol or butyl carbitol, and theorganic vehicle may be ethyl cellulose. The high dielectric constantglass may be lead-germinate or zinc borate glass. Preferably, thesurfactant is a phosphate ester.

The process of the instant invention results in an internal barrierlayer ultracapacitor (IBLC) made from novel dielectric materials as abattery replacement with the following advantages: longer life, lowermass-to-weight ratio, rapid charging, on-demand pulse power, improvedstandby time without maintenance, and environmental friendliness.

Test pellets were fabricated utilizing BaTiO₃ particles of 730 and 500nm particle sizes. Some of the test pellets were made with BaTiO₃particles that were coated with SiO₂, and test pellets were fabricatedutilizing BaTiO₃ particles (500 nm) that were coated with Al₂O₃. TheSiO₂ coating thickness was 5 nm, and the Al₂O₃ thickness was 10 nm.

BaTiO₃ particles were coated using an atomic layer deposition (ALD)process. Atomic Layer Deposition (ALD) is a thin film deposition methodin which a film is grown on a substrate by exposing its surface toalternate gaseous species (typically referred to as precursors). Incontrast to chemical vapor deposition (CVD), the precursors are neverpresent simultaneously in the reactor, but they are inserted as a seriesof sequential, non-overlapping pulses. In each of these pulses theprecursor molecules react with the surface in a self-limiting way, sothat the reaction terminates once all the reactive sites on the surfaceare consumed. Consequently, the maximum amount of material deposited onthe surface after a single exposure to all of the precursors (aso-called ALD cycle) is determined by the nature of theprecursor-surface interaction. See Puurunen, Riikka, Surface chemistryof atomic layer deposition: A case study for the trimethylaluminum/waterprocess, Journal of Applied Physics 97, 121301 (2005).

By varying the number of cycles it is possible to grow materialsuniformly and with high precision on arbitrarily complex and largesubstrates. The un-sintered particles were pressed into pellets withoutthe addition of binder using a potassium bromide dye, and a tube furnacewas used to heat the pellets under an atmosphere of 96% N₂ and 4% H₂. Insuch an atmosphere, BaTiO₃ is slightly reduced. Quartz boats were eachpopulated with pellets of Al₂O₃-coated, SiO₂-coated, and uncoatedBaTiO₃. After processing and the pellets were left to cool to roomtemperature inside the tube furnace. Resulting pellets were 4-8 mm thickwith masses of 1.5-2.5 g.

Capacitors can also be made by 3D additive manufacturing. To perform 3Dadditive manufacturing, the particles are first converted into an ink.Two additive manufacturing techniques used for electrode and dielectricdeposition, such as aerosol jet deposition and screen printing, requireunfused particles in order to deposit the material properly. In order todeposit the particles, they are separated using a three-roll mill orsimilar machine.

The aerosol jet process begins with a mist generator that atomizes asource material. Particles in the resulting aerosol stream are thencondensed. The aerosol stream is then aerodynamically focused using aflow guidance deposition head, which creates an annular flow of sheathgas to collimate the aerosol. The co-axial flow exits the flow guidancehead through a nozzle directed at the substrate, which serves to focusthe material stream to as small as a tenth of the size of the nozzleorifice (typically 100 μm).

The aerosol jet process allows for a large viscosity range ofprocessible inks (typically 0.7-2,500 cP), a flexible distance betweensubstrate and nozzle (typically 1 to 5 mm) as well as a tightly focusedaerosol stream for variable line width. This allows the production offine pitch (typically below 50 μm) electronic devices. Machines forperforming this process are available from Optomec, Inc., New Mexico;under the brand name Aerosol Jet®.

Screen printing is the process of using a mesh-based stencil to applyink onto a substrate, whether it be T-shirts, posters, stickers, vinyl,wood, or other material.

Some areas of the mesh are made impermeable and the mesh placed over thesubstrate. A blade or squeegee is used to move ink across the screen tofill the open mesh apertures with ink. A reverse stroke then causes thescreen to touch the substrate momentarily along a line of contact. Thiscauses the ink to wet the substrate and be pulled out of the meshapertures as the screen springs back after the blade has passed.

Screen printing was utilized to fabricate test cells. The capacitor testcells were sintered using a belt furnace after each layer deposition.Subsequent testing showed energy densities in the range of 1.0 to 2.0J/cc.

An appreciation of the other aims and objectives of the presentinvention and an understanding of it may be achieved by referring to theaccompanying drawings and description of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 Schematic of a conventional capacitor

FIG. 2A Schematic of an EDLC

FIG. 2B Enlargement of area marked with a B on FIG. 2A

FIG. 3 Ragone chart of energy storage devices

FIG. 4A Internal barrier layer capacitor

FIG. 4B Enlargement of area identified on FIG. 4A

FIG. 4C Internal barrier layer capacitor effect

FIG. 5A BaTiO₃ particle with 10 nm of Al₂O₃ coating.

FIG. 5B BaTiO₃ particle with SiO₂ coating

FIG. 6 The BaTiO₃ crystal structure. The green, red, and blue atoms aretitanium, oxygen, and barium, respectively

FIG. 7 Dielectric Test Fixture 1645-1B in front of Agilent E4980Aprecision LRC meter.

FIG. 8A Top view of ultracapacitor cell

FIG. 8B Side view of ultracapacitor cell

FIG. 8C Layer view of ultracapacitor cell

FIG. 9 Eight-zone belt furnace temperature settings. Zones 1-8 are theindividual heated zones; PV=present temperature value; FV=future, ordesired temperature value for the profile; and SV=set temperature valuefor the individual heater zone controller.

FIG. 10 Belt furnace temperature profile

FIG. 11A SEM image of uncoated, untreated BaTiO₃

FIG. 11B SEM image of BaTiO₃ coated with Al₂O₃ but untreated.

FIG. 11C SEM image of BaTiO₃ coated with Al₂O₃ and treated at 750° C.for 30 hr.

FIG. 12A Optical microscopy photographs of (a) uncoated BaTiO₃ pellets(untreated)

FIG. 12B Optical microscopy photographs of coated, BaTiO₃ pellets(untreated)

FIG. 12C Optical microscopy photographs of Al₂O₃ coated BaTiO₃ pellets(untreated)

FIG. 13A Optical microscopy photographs of uncoated, BaTiO₃ pelletstreated at 900° C. for 1 hr.

FIG. 13B Optical microscopy photograph of SiO₂ coated BaTiO₃ pelletstreated at 900° C. for 1 hr.

FIG. 13C Optical microscopy photographs of Al₂O₃ coated BaTiO₃ pelletstreated at 900° C. for 1 hr.

FIG. 14A Optical microscopy photographs of uncoated, BaTiO₃ pelletstreated at 1,100° C. for 1 hr.

FIG. 14B Optical microscopy photographs of SiO₂ coated BaTiO₃ pelletstreated at 1,100° C. for 1 hr.

FIG. 14C Optical microscopy photographs of Al₂O₃ coated BaTiO₃ pelletstreated at 1,100° C. for 1 hr.

FIG. 15A Plots of permittivity of samples treated at 900° C. for 15 hr.and 1,100° C. for 1 hr. compared to the untreated powders

FIG. 15B Plots of DF of samples treated at 900° C. for 15 hr. and 1,100°C. for 1 hr. compared to the untreated powders

FIG. 15C Plots of ESR of samples treated at 900° C. for 15 hr. and1,100° C. for 1 hr. compared to the untreated powders

FIG. 16A Plots of permittivity of samples treated at 900° C. for 1 hr.compared to the untreated powders

FIG. 16B Plots of DF of samples treated at 900° C. for 1 hr. compared tothe untreated powders

FIG. 16C Plots of ESR of samples treated at 900° C. for 1 hr. comparedto the untreated powders

FIG. 17 Ultracapacitor test cell made from SiO₂-coated BaTiO₃ depositedby screen printing

FIG. 18A Plots of permittivity of powdered samples treated at 900° C.for 1 hr. before and after furnace sintering

FIG. 18B Plots of DF of powdered samples treated at 900° C. for 1 hr.before and after furnace sintering

FIG. 18C Plots of ESR of powdered samples treated at 900° C. for 1 hr.before and after furnace sintering

FIG. 19A SEM image at a magnification of 500 showing the level ofdensification of SiO₂-coated BaTiO₃ test cell with 184 nF of capacitanceprocessed at 900° C. for 1 hour.

FIG. 19B SEM image at a magnification of 3,000 showing the level ofdensification of SiO₂-coated BaTiO₃ test cell with 184 nF of capacitanceprocessed at 900° C. for 1 hour.

FIG. 19C SEM image at a magnification of 5,000 showing the level ofdensification of SiO₂-coated BaTiO₃ test cell with 184 nF of capacitanceprocessed at 900° C. for 1 hour.

FIG. 19D SEM image at a magnification of 10,000 showing the level ofdensification of SiO₂-coated BaTiO₃ test cell with 184 nF of capacitanceprocessed at 900° C. for 1 hour.

FIG. 20 Voltage versus time used for the direct current (DC) dischargetest method.

FIG. 21A Plots of capacitance of the capacitor test cells, made from thedielectric material treated at 900° C. for 1 hr., before and afterfurnace sintering

FIG. 21B Plots of DF of the capacitor test cells, made from thedielectric material treated at 900° C. for 1 hr., before and afterfurnace sintering

FIG. 21C ESR of the capacitor test cells, made from the dielectricmaterial treated at 900° C. for 1 hr., before and after furnacesintering

FIG. 22 Nine, multilayered ultracapacitor cells in parallel printed on asubstrate board. Cells can be printed serially or in parallel to get thedesired voltage or capacitance. This is known as a slice.

FIG. 22A is a cross section of a multilayer capacitor cell, nine ofwhich are used in a slice.

FIG. 23 Ultracapacitor module where multilayered capacitor boards (orslices) are stacked in a housing with active or passive cooling toincrease energy storage.

FIGS. 24A-24D show frequency dependence of Cp˜D and ∈_(r) for 2different samples at room temperature from 0.1 Hz to 1 MHz.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope thereof and additional fields in which the presentinvention would be of significant utility.

The instant invention is a method of manufacturing an IBLC capacitor 38.Particles of BaTiO₃ 40 (see FIGS. 4, 5 and 6) having an average graindiameter of 100-700 nm are first heated in a furnace under a mixture of70-96% by volume N₂ 4-30% by volume H₂ gas for 60-90 minutes at 850-900°C.

BaTiO₃ has doubly ionized oxygen anion vacancies—see FIG. 6. The firstfurnace may be a multizone belt furnace or a fluidized bed vertical tubefurnace. The second furnace may be a multizone belt furnace.

Next a 3-10 nm film of SiO₂ 48 or Al₂O₃ 44 is deposited over eachindividual particle 40. The resulting grains 32 (see FIGS. 5A and 5B)are agglomerated so they must be mechanically separated. The grains arethen incorporated into an ink of the following formulation:

-   -   i. 60-80% by weight separated grains 32;    -   ii. 5-50% by weight high dielectric constant glass; the high        dielectric constant glass being 1-10 μm in size    -   iii. 0.1-5% by weight surfactant;    -   iv. 5-25% by weight solvent; and    -   v. 5-25% weight organic vehicle.

The solvent may be ester alcohol, terpineol or butyl carbitol, and theorganic vehicle may be ethyl cellulose. The high dielectric constantglass may be lead-germinate or zinc borate glass. Preferably, thesurfactant is a phosphate ester.

Next a layer of the dielectric ink 24 a is deposited on a substrate 62with a pre-sintered and deposited electrode 60 in place. The electrodecan be silver, silver palladium, or any material with a resistancebetween 1 milliohm and 10 ohms. Finally, the dielectric ink 24 a issintered onto the substrate 62 by heating in a second furnace at850-900° C. for less than 5 minutes and allowing the ink 24 a andsubstrate 62 to cool to ambient. This heating and cooling cycle iscarried out under N₂ atmosphere, which contains less than 25 ppm 02.Preferably, during sintering, the time under 600° C. is kept to 30minutes maximum; time under 800° C. is 20 minutes maximum; and totaltime is 60-90 minutes. Also, preferably, the heating rate is 45-55°C./minute from 300-500° C.; and the cooling rate is 45-55° C./minutefrom 700-300° C. Once the dielectric is sintered, a top electrode 74 isadded that can be silver, silver palladium or any material with aresistance between 1 milliohm and 10 ohms.

The substrate 62 is preferably 0.025-0.040 inch thick Al₂O₃ in which theAl₂O₃ is at least 96% pure. Alternatively, the substrate could bealuminum nitride (AlN) zirconia, beryllium oxide (BeO) or uncuredceramic. Uncured ceramic is a mixture of ceramic particles, a binder, asurfactant and a solvent.

Preferably, the thickness of the deposited dielectric ink 24 a issufficient to produce a sintered layer 10-35 μm thick.

The process of the instant invention results in an internal barrierlayer ultracapacitor (IBLC) 38, which can be used as a batteryreplacement because it has the following advantages: longer life, lowermass-to-weight ratio, rapid charging, on-demand pulse power, improvedstandby time without maintenance, and environmental friendliness.

This invention is also an ink of the formula shown above.

Experimental I

Test pellets were fabricated utilizing BaTiO₃ particles 40 of 730 and500 nm particle sizes. Some of the test pellets were made with BaTiO₃particles 40 that were coated with SiO₂ 48, and test pellets werefabricated utilizing BaTiO₃ particles 40 (500 nm) that were coated withAl₂O₃ 44. The SiO₂ coating 48 thickness was 5 nm, and the Al₂O₃ 44thickness was 10 nm.

BaTiO₃ particles 40 were coated using an atomic layer deposition (ALD)process.

Some un-sintered particles were pressed into pellets without theaddition of binder using a potassium bromide dye. A tube furnace wasused to heat the pellets under an atmosphere of 96% N₂ and 4% H₂. Insuch an atmosphere, BaTiO₃ 40 is slightly reduced. Quartz boats wereeach populated with pellets of Al₂O₃-coated, SiO₂-coated, and uncoatedBaTiO₃ 40. After processing the pellets were left to cool to roomtemperature inside the tube furnace. Resulting pellets were 4-8 mm thickwith masses of 1.5-2.5 g.

Capacitors can also be made by 3D additive manufacturing. To perform 3Dadditive manufacturing, the particles are first converted into an ink.Two additive manufacturing techniques can be used for dielectric 78deposition, such as aerosol jet deposition and screen printing. Theyrequire unfused particles in order to deposit properly. In order toscreen print the particles, they are separated using a three-roll millor similar machine.

The aerosol jet process begins with a mist generator that atomizes asource material. Particles in the resulting aerosol stream arecondensed. The aerosol stream is then aerodynamically focused using aflow guidance deposition head, which creates an annular flow of sheathgas to collimate the aerosol. The co-axial flow exits the flow guidancehead through a nozzle directed at the substrate, which serves to focusthe material stream to as small as a tenth of the size of the nozzleorifice (typically 100 μm).

The aerosol jet process allows for a large viscosity range ofprocessible inks (typically 0.7-2,500 cP), a flexible distance betweensubstrate and nozzle (typically 1 to 5 mm) as well as a tightly focusedaerosol stream for variable line width. This allows the production offine pitch (typically below 50 μm) electronic devices. Machines forperforming this process are available from Optomec, Inc., New Mexico;under the brand name Aerosol Jet®.

Screen printing is the process of using a mesh-based stencil to applyink onto a substrate, whether it be T-shirts, posters, stickers, vinyl,wood, or other material.

Some areas of the mesh are made impermeable and the mesh placed over thesubstrate. A blade or squeegee is used to move ink across the screen tofill the open mesh apertures with ink. A reverse stroke then causes thescreen to touch the substrate momentarily along a line of contact. Thiscauses the ink to wet the substrate and be pulled out of the meshapertures as the screen springs back after the blade has passed.

Screen printing was utilized to fabricate test cells. The capacitor 38test cells were sintered using a belt furnace after deposition of eachlayer. Subsequent testing showed energy densities in the range of 1.0 to2.0 J/cc.

Atomic Layer Deposition-Coated Ceramic Barium Titanate Particles 40

A prior study focused on BaTiO₃ particles 40 of various sizes in bothcoated and uncoated configurations, with the latter serving as abaseline. Table 3 provides the details on particle diameter, coatingmaterial and thickness, purity, and supplier.

TABLE 3 Materials. Particle Thick- Size Purity ness Material Supplier(nm) (%) Coating (nm) Color BaTiO₃ 40 Ferro 730 99.95 Uncoated — WhiteBaTiO₃ 40 TPL 500 99.95 SiO₃  5 Light grey BaTiO₃ 40 ALD 500 99.95 Al₂O₃10 White Nano- Solutions BaTiO₃ 40 Sakai 140 99.95 uncoated — white

The BaTiO₃ particles 40 used in this study varied in diameters rangingfrom 140 nm to 730 nm as their D50, or median particle size. Coatingconfigurations varied from uncoated to 10 nm. The uncoated BaTiO₃ 40sample was a fine powder, while the coated BaTiO₃ 40 samples wereagglomerated. The clumps are likely caused by hydrophilic interaction orstatic charge. The clumps were dispersed before processing into inkformulations.

Atomic layer deposition (ALD) was used to deposit nanothin films overBaTiO₃ nanoparticles. The nanothin film coatings consist of a 10 nmthick layer of alumina (Al₂O₃ 44) and 5 nm thick layer of silica (SiO₂48). FIG. 5A illustrates a BaTiO₃ particle 40 coated with Al₂O₃ 44. FIG.5B illustrates a BaTiO₃ particle 40 coated with SiO₂ 48. The number ofcycles performed during ALD determines the coating thickness. Thecoating thickness rate for Al₂O₃ 44 was 10 Å per cycle, and for SiO₂ 48was 4 Å per cycle. It is important to note that previous IBLC researchusing SPS of BaTiO₃ particles 40 were coated by the Stöber process, amethod based on a seeded growth process. The Stöber process is known toproduce an inconsistent coating.

High-Temperature and Reduced Forming Gas Sintering

In reducing atmospheres (75-96% N₂ and 4-25% H₂), BaTiO₃ 40 is slightlyreduced, forming doubly ionized oxygen (anion) vacancies. This producesthe same effect as vacuum sintering, so a reducing atmosphere is thepreferred method of processing. To understand vacancy creation, BaTiO₃40 crystal structure is shown in FIG. 6. The conductivity results fromthe electron exchange between Ti⁺⁴ and Ti⁺³ resulting from oxygenvacancies at the octahedron. The induced free electrons make the reducedperovskite material highly semiconducting as shown in equations (10) and(11). Sintering BaTiO₃-based dielectrics in forming gas decreases theinsulation resistance by 10-12 orders of magnitude. FIG. 6 shows theBaTiO₃ crystal structure.BaTiO₃ +xH₂→=BaTiO_(3-x)[V₀]_(x) +xH₂O  (10)and[V₀]→=[V₀]⁻+2e  (11)

A three-zone, Thermo Scientific™ Lindberg/Blue tube furnace was used toprocess the particles. The furnace was heated to 850-950° C. for atleast 60 minutes. The uncoated BaTiO₃ 40, serving as a baseline, wasalways heat treated to evaluate its electrical properties versus thoseof coated particles. The forming gas was turned on at 1-3 SCFH for 10min. prior to placing the samples inside. After the desired annealingduration, the forming gas was left flowing until the powder reached atemperature under 300° C. to avoid any reoxidation of the powder. Thesamples were left to cool to room temperature inside the tube furnacebefore removal.

Previous studies show that the reduction of BaTiO₃ 40 in H₂ atintermediate temperatures (500° C.) leads to bodies of bright yellowcolor. Reduced SiO₂-coated material obtained through SPS at a finaltemperature of 1,110° C. is expected to change from white to a navy bluecolor. Uncoated BaTiO₃ 40 and doped BaTiO₃ specimens that show aremarkable reduction in resistivity has also been characterized with abluish color. To assess color changes, optical microscopy images of thepellets were taken at ×7 magnification.

When powdered particles 32 are heated to a high temperature below themelting point, the atoms in the particles diffuse across the particleboundaries, fusing the particles together. Two additive manufacturingtechniques used for electrode and dielectric deposition, such as aerosoljet deposition and screen printing, require unfused particles in orderto deposit the material properly.

In order to screen print the particles, they were separated using athree-roll mill.

Pellet Electrical Characterization

Un-sintered BaTiO₃ particles 40 were pressed into pellets without theaddition of binder using a potassium bromide die. A literature reviewrevealed that pellets pressed at pressures above 345 MPa (50,000 psi)could not be recovered. Various pressures were tested, revealing thatpellets pressed at forces above 1.8 kN (400 lb.) could not be recoveredfrom the potassium bromide die in suitable shape. Because of thesefindings, the pellets were made by pressing them at 1.3 kN (300 lb.) offorce using a TestResources (Shakopee, Minn.) compression and tensionmachine. The pellets were 4-8 mm thick with masses of 1.5-2.5 g.

Adsorption of water vapor increases the permittivity by a factor of 2.19However, the focus of the characterization at this phase of the studywas to identify a sample with a large change in permittivity,specifically by a factor of 10⁴. Because the focus was large changes inpermittivity, no attempt was made to remove water. In addition, thinfilm electrical characterization is used to obtain the most accuratemeasurements, and since these samples are sintered, water absorptioneffects are eliminated.

Capacitance, DF, and ESR were measured for a frequency range of 20 Hz to2 MHz using a Dielectric Test Fixture 1645-1B together with an AgilentE4980A precision inductance, capacitance, and resistance (LCR) meter,shown in FIG. 7. The capacitance was initially assumed small, andtherefore, measurements were made using the LCR meter's parallel mode.If the values were found to be higher than expected, then the instrumentcould be reset to use series mode. The dielectric constant of thesamples was determined from the instrument's reported capacitance value.No porosity correction was made to the dielectric constant. FIG. 7 showsAgilent E4980A precision LCR meter (top) and Dielectric Test Fixture1645-1B (bottom).

Dielectric Ink Formulation

To perform 3D additive manufacturing, the powders were first convertedinto an ink. The formulation for this ink is shown in table 4. Glassparticulates were used to increase densification, but high quantities ofglass particles decrease the permittivity, so the concentration of glasswas kept as low as possible to produce a usable ink. Surfactant was usedas a wetting agent to allow the ink to spread. A thinner was also usedto get the proper ink viscosity. Texanol™ was used as a thinner becauseit volatilizes at 120° C. The vehicle was an organic binder formulatedfrom a blend of Ashland Chemical ethyl cellulose in Texanol estersolvent. It was used to further enhance the viscosity of the ink. Thevehicle was chosen because it volatilizes between 250° C. and 350° C.during sintering.

TABLE 4 Dielectric ink formulation. Component Concentration (%) BaTiO₃dielectric 32 72.5 Lead-germinate high K glass 7.5 Surfactant (wettingagent) 0.5 Texanol (solvent) 5 Ethyl cellulose organic vehicle 15

The dielectric ink 24 a formulation was mixed and then ground in athree-roll mill. A three-roll mill is a tool that uses shear force bythree horizontally positioned rolls rotating at opposite directions anddifferent speeds relative to each other to mix, refine, disperse, andhomogenize viscous materials fed into it. The final ink was a dense,homogenous mixture used for screen printing.

Three-Dimensional Additive Thin Film Deposition

The screen printing method was the chosen method of printing a test cellfor this study. This technique can produce layers as thin as 5 μm. Byproducing such a thin dielectric layer, the capacitance equation showsthat the energy stored can be increased significantly. The screenprinting process began by creating a capacitor design on a woven meshusing photolithography. The ink was forced into the mesh openings by asqueegee and onto the printing surface during the squeegee stroke. Thelarger the number of intertwined meshes, the thinner the depositionbecame for a single stroke. The capacitor layers (FIGS. 8A-8C) wereprinted using a Hary Manufacturing, Inc. (Lebanon, N.J.) 485 precisionscreen printer. Palladium silver ink that is used in multilayer chipcapacitors due to its conductance and resistance to silver migration wasused as the electrode 60, 74 material. Al₂O₃ (0.039 in and 96% purity)was used as the substrate 62 on which each layer was deposited. Al₂O₃was chosen because it has a very low coefficient of expansion and willnot impart excessive stress during later sintering steps. As a result,each layer is only able to densify in the z-axis due to clamping to thesubstrate 62. The ultracapacitor test cells 38 were made using twolayers of dielectric ink 24 a applied through 325 and 400 mesh screens.FIGS. 8A-8C shows views of the ultracapacitor 38 cell: 8A Top, 8B side,and 8C layers

The capacitor test cell was sintered using a HAS 1505-0811Z belt furnacefrom HengLi Eletek Co. (San Diego, Calif.) at 850° C. peak for 10 minand a total cycle time of 1.5 hr. This sintering step was performedafter each layer of deposition in order to burn off organic materialsand achieve high densification. The temperature settings of theeight-zone belt furnace are shown in FIG. 9, the temperature profile inFIG. 10, and the N₂ flow profile in table 5. Previous work shows thatwhen reduced SiO₂-coated 48 BaTiO₃ 40 is post-annealed at 800° C. for 12hr. in air, it remains blue, while reduced uncoated BaTiO₃ 40 turnswhite. For this reason, the SiO₂ shell 48 is thought to act as anefficient barrier against oxidation. As a further preventative measure,the belt muffle furnace was purged with N₂ to avoid re-oxidation.Densification of the dielectric layer was then evaluated with thescanning electron microscope (SEM). FIG. 9 shows temperature settingsfor the eight-zone belt furnace. On FIG. 9, zones 1-8 are the individualheated zones; PV stands for present temperature value; FV stands forfuture, or desired temperature value for the profile; and SV stands forset temperature value for each individual zone heater controller. FIG.10 shows the belt furnace temperature profile.

TABLE 5 Furnace N₂ flow profile. Section Nitrogen Flow (LPM) Entrancecurtains 40 Preheat 45 Venturi exhaust 100 Cooling gas 20 Exit curtains20

Thin Film Electrical Characterization

The ultracapacitor 38 test cell was measured for parallel capacitanceusing an LCR meter. Capacitance readings were then used to determine ifthe device was functional. Samples that showed functionality were alsotested via the discharge method. To use the discharge method, thecapacitor was discharged through a resistor that was chosen to yield areasonable time constant. The voltage versus time plot was captured witha DP05104 digital phosphor oscilloscope (Marietta, Ga.). A large regionof the discharge curve was chosen, and the values of voltage in thedischarge cycle and time required to drop between the two voltages wereentered into equation (12) along with the known resistor value. In thisequation, t is the time it takes to discharge the capacitor between someinitial voltage (V) to some final volt-age (Vf). The capacitance (C) isto be determined, and R is a resistor through which the capacitor isdischarged:

$\begin{matrix}{t = {C*R*{{\ln\left( \frac{V_{f}}{V_{i}} \right)}.}}} & (12)\end{matrix}$

Analysis

Pellet Electrical Characterization

SEM images of the untreated particles 40 (FIGS. 11A and 11B) revealedthat particles indicated by the manufacturer to be 500 nm actuallyvaried in diameter from 250 nm up to 1 μm. Treated particles 32 (FIG.11C) also showed varying particle sizes of the same range. Theseobservations show that the furnace treatment was not causing large scalegrain growth. FIGS. 11A-11C show SEM images of BaTiO₃ 40: 11A uncoated,11B coated with Al₂O₃ 44, and 11C Al₂O₃ 44 coated, treated at 750° C.for 30 hr.

All three batches of particles 32, 12 were initially white in color, ascan be seen in FIGS. 12A-12C. When treated at temperatures below 900°C., they turned to a bright yellow or a neon green color. Theseparticles remained that color under the reducing forming gas atmosphereand changed to white after the first minute of exposure to air. Scrapingoff the top layer of the treated powder revealed two shades of color: alighter tone on top and a darker tone underneath. This non-uniformcolor, shown in FIGS. 13 and 14, indicates that the particles were notbeing reduced homogeneously. This indicated that proper reduction of theparticles had to be done individually in a fluidized bed as opposed topelletized form and this treatment method was adopted. FIGS. 12A-12Cshow optical microscopy photographs: 12A uncoated, 12B SiO₂-coated 48,and 12C Al₂O₃ 44 coated BaTiO₃ 40 pellets (untreated). FIGS. 13A-13Bshow optical microscopy photographs: 13A uncoated, 13B SiO₂-coated 438,and 13C Al₂O₃ 44 coated BaTiO₃ 40 pellets treated at 900° C. for 1 hr.FIGS. 14A-14C show optical microscopy photographs: 14A uncoated, 14BSiO₂-coated 48, and 14C Al₂O₃ 44 coated BaTiO₃ 40 pellets treated at1,100° C. for 1 hr.

At temperatures below 900° C., no significant changes were seen in thepermittivity. At temperatures above 900° C., the permittivity and DFslightly increased for uncoated BaTiO₃ 40 and decreased for coatedsamples 44, 48. The ESR decreased only for the Al₂O₃ 44 coated sample,the greatest decrease occurring with 900° C. treatment. The decrease inESR seen in FIGS. 15A-15B coincides with the color change seen in FIGS.14A-14B. This can be interpreted as the material undergoing reduction.

The synthesis conditions that produced the maximum increase inpermittivity for all samples was at 900° C. for 1 hr. Table 6 shows theeffect of a short-duration treatment versus a long-duration treatmentwith constant (900° C.) temperature. The SiO₂-coated sample exhibits thehighest permittivity.

TABLE 6 Synthesis profile effect on dielectric permittivity. At 20 HzUntreated 1 hr. at 900° C. 15 hr. at 900° C. BaTiO₃ 40 ColorPermittivity Color Permittivity Color Permittivity Uncoated 40 White 9White 2,227 White 708 Al₂O₃- White 217 Light 6,886 Navy 182 coated 44blue blue SiO₂- White 7,638 Grey 19,980 Grey 4,384 coated 48

The capacitor properties versus frequency of the samples treated at 900°C. for 1 hr. are compared in FIGS. 16A-16B. Low-frequency permittivitiesare high (maximum: 19,980 at 20 Hz), indicating the dielectric would begood for DC applications. The DF was found to increase with treatment.The decreased ESR for all treated powders indicated that they arebecoming semiconducting, one of the desired outcomes for the IBLCeffect. SiO₂-coated 48 and Al₂O₃ 44 coated BaTiO₃ 40 treated at 900° C.for 1 hr. SiO₂-coated 48 and Al₂O₃-coated 44 BaTiO₃ 40 treated at 900°C. for 1 hr were chosen as the dielectric for the capacitor test cellbecause of they had the best capacitance traits.

Thin Film Electrical Characterization

FIG. 17 shows an ultracapacitor test cell made with SiO₂-coated 48BaTiO₃ 40.

SEM images (FIG. 19) of the ultracapacitor 38 test cells showed a70%-80% densification. The thickness for samples built using the 325mesh was an average of 20 μm. The capacitance obtained through thedischarge method using a DC source agreed with the LCR measuredcapacitance values within ±5%. The voltage versus time plot for the184.2 nF capacitor is shown in FIG. 20.

FIG. 20 shows voltage versus time used for the discharge method.

Electrical characterization (FIGS. 21A-21C) shows normal capacitorbehavior up to 1.3 MHz. Above the latter frequency, the capacitor testcells exhibit a negative capacitance and a DF that spikes up to 3×103.This negative capacitance effect is observed in a variety ofsemiconductor devices.

Experimental II

Several capacitors 38 were made as described. An Agilent (Santa Clara,Calif.) 4294A impedance analyzer was used to characterize thedielectric/electric properties of these devices over a frequency rangefrom 100 Hz to 100 MHz using Cp˜D and R˜X function. 301 points werechosen in this range.

A Solartron (West Sussex, UK) SI 1260 Impedance/Gain Phase Analyzer wasused for the low frequency characterization from 0.1 Hz to 10 kHz atroom temperature. In the experiments, the AC amplitude is a constant of100 mV, while the DC bias is 0 V. 50 points were chosen in this range.

The P-E hysteresis loops were measured using Sawyer-Tower circuit(Radiant Technologies Precision LC unit, Albuquerque, N. Mex.). Theprofile is standard bipolar and frequency is 10 Hz.

FIGS. 24A-24B show the two results combined: 1) high frequency usingAgilent 4294A impedance analyzer (100 Hz˜1 MHz) (open dot) and 2) Lowfrequency using Solartron SI 1260 Impedance/Gain Phase Analyzer (0.1Hz˜10 kHz) (solid line). The capacitance of ALD#1 is about 500 nF. Thecapacitance of ALD#2 is about 600 nF.

CONCLUSIONS

A material and set of processing conditions were selected that gave theoptimal properties for fabricating a capacitor 38. The material ofchoice, SiO₂-coated 48 BaTiO₃ 40, exhibited the highest dielectricpermittivity. This particular sample was treated at 900° C. for 1 hr.The processed material exhibited the following properties at 20 Hz:permittivity of 19,980, a DF of 215%, and an ESR of 806 kOhms. A testcell was built with the selected material at a thickness of 13.5 μm, andit exhibited a capacitance of 125 nF at 1 kHz. The breakdown voltage ofthis sample was measured to be 450V. The calculated energy density basedon a 184 nF capacitor at this breakdown voltage would be about 5 J/cc.Treatment at temperatures below 900° C. does not significantly affectthe dielectric properties of the material. The decrease in propertiesfor samples treated above 900° C. may be attributed to an over reductionor to excess inter-diffusion. SiO₂, although it did not experience acolor change, had the highest initial and after treatment permittivity.The color tone difference within a powder batch after being reducedindicates that a better sealed tube furnace or other synthesistechniques like the fluidized bed process, are necessary to obtain ahomogeneous treatment.

The following reference numbers are used on FIGS. 1-24.

-   -   4 positive electrode    -   6 air gap capacitor    -   8 resistive load    -   10 electrolytic capacitor    -   12 applied voltage    -   16 current flow    -   20 negative electrode    -   22 electrolyte    -   24 air dielectric    -   24 a dielectric ink    -   26 separator    -   28 current collector    -   32 grain of conductive ceramic    -   36 capacitive grain boundary    -   38 IBLC capacitor    -   40 BaTiO3 particle    -   44 Al2O3 coating    -   48 SiO2 coating    -   52 soldered connection to test lead    -   56 connection soldered to bottom electrode    -   60 bottom electrode    -   62 substrate    -   64 bottom contact pad    -   66 test lead    -   70 connection soldered to top electrode    -   74 top electrode    -   76 top contact pad    -   78 dielectric made in accordance with this invention    -   82 multilayer capacitor cell    -   84 multilayer capacitor slice    -   86 housing    -   90 cooling block

Thus, the present invention has been described herein with reference toa particular embodiment for a particular application. Those havingordinary skill in the art and access to the present teachings willrecognize additional modifications, applications and embodiments withinthe scope thereof

It is therefore intended by the appended claims to cover any and allsuch applications, modifications and embodiments within the scope of thepresent invention.

What is claimed is:
 1. A method of manufacturing a capacitor comprisingthe steps of: a) obtaining BaTiO₃ particles; said particles having anaverage grain diameter of 100-700 nm; b) treating said particles in afirst furnace under a mixture of 70-96% by volume N₂ and 4-30% by volumeH₂ gas for 60-90 minutes at 850-900° C.; c) coating said treatedparticles with a 3-20 nm thick film of SiO₂ or a 3-10 nm thick film ofAl₂O₃ whereby said coated treated particles become agglomerated; d)separating said coated, treated particles to break up said agglomerationinto individual particles; e) incorporating said separated, coated,treated particles into an ink comprising: i) 60-80% by weight separated,coated, treated particles; ii) 5-50% by weight high dielectric constantglass; said high dielectric constant glass being 0.5-10 μm in size; iii)0.1-5% by weight surfactant; iv) 5-25% by weight solvent; and v) 5-25%by weight organic vehicle; f) depositing an electrode on a substrate;said electrode having a resistance between 1 mΩ and 10Ω; g) sinteringsaid substrate and electrode; h) depositing a layer of said ink on saidsubstrate and electrode by a deposition process so that said layer iscontinuous and has consistent thickness; i) removing solvent from saidlayer by drying for 15-30 minutes at 120-150° C. in air; j) repeatingsteps h) and i) until desired thickness is obtained; k) removing organicbinder by exposing said substrate, electrode and ink to the followingheating cycle: i) gradually increasing temperature to 280-350° C. over45-90 minutes with a heating rate not exceeding 10-15° C. per minute ii)curing at 280-350° C. for 4-72 hours; iii) allowing said substrate,electrode and ink to cool to ambient temperature; l) sintering said atleast one layer on said substrate by heating in a second furnace, at850-900° C. for less than 5 minutes and allowing it to cool to ambientunder N₂ atmosphere; said N₂ containing less than 25 ppm O₂; a thicknessof said at least one layer being sufficient to produce a sintered layer10-35 μm thick; and m) depositing a top electrode on said at least onelayer; said top electrode having a resistance between 1 mΩ and 10Ω.
 2. Amethod as claimed in claim 1 in which said coating process is atomiclayer deposition.
 3. A method as claimed in claim 1 in which saiddeposition process of step h) is aerosol jet deposition or silkscreening or inkjet printing.
 4. A method as claimed in claim 1 in whichsaid separating is performed by a three roll mill or a high shear mixer.5. A method as claimed in claim 1 in which said high dielectric constantglass is lead-germinate glass or zinc borate glass.
 6. A method asclaimed in claim 1 in which said surfactant is a phosphate ester.
 7. Amethod as claimed in claim 1 in which said solvent is ester alcohol,terpineol or butyl carbitol.
 8. A method as claimed in claim 1 in whichsaid substrate is 0.038-0.040 inch thick and at least 96% pure Al₂O₃,aluminum nitride (AlN), zirconia, beryllium oxide (BeO) or uncuredceramic.
 9. A method as claimed in claim 8 in which said uncured ceramicis a mixture of ceramic particles, a binder, a surfactant and a solvent.10. A method as claimed is claim 9 in which said solvent is2,2,4-trimethyl-1,3-pentandiol monoisobutyrate, terpineol (C₁₀H₁₈O) orbutyl carbitol.
 11. A method as claimed is claim 9 in which saidsurfactant is a phosphate ester.
 12. A method as claimed in claim 1 inwhich said organic vehicle is ethyl cellulose.
 13. A method as claimedin claim 1 in which said first furnace is a multizone belt furnace. 14.A method as claimed in claim 1 in which said first furnace is afluidized bed vertical tube furnace.
 15. A method as claimed in claim 1in which said second furnace is a multizone belt furnace.
 16. A methodas claimed in claim 1 in which during sintering in step I), time under600° C. is 30 minutes maximum; time under 800° C. is 20 minutes maximum;and total time is 60-90 minutes.
 17. A method as claimed in claim 14 inwhich during sintering in step I), the heating rate is 45-55° C./minutefrom 300-500° C.; and the cooling rate is 45-55° C./minute from 700-300°C.